The present invention relates to telecommunications networks and in particular to a telecommunications network implementing over the horizon communications using the very high frequency (VHF) and/or the ultra high frequency (UHF) spectrum of the communications band.
Numerous telecommunications architectures exist or have been proposed for adding bandwidth and/or capacity to current communications systems. For example, fiber optic cable networks/ISDN telephone lines are the current state of the art in providing high capacity voice and data services using landlines. The advantages of this transmission medium are only available where a hardwired connection exists and a physical network has been installed. This restriction is true even for the lower bandwidth analog telephone and cable systems also in popular use. Therefore, a mechanism must exist between these subnetworks when a voice/data message transmits from a source network to a destination network not physically connected with each other.
In current applications, various mechanisms are used to link between the hardwired subnetworks that connect directly to the end user. Where line of sight communications are available, microwave towers and UHF repeaters are often used to relay transmission to a receiving antenna or other microwave routing facility. Optionally, the communication is routed to a satellite based communications network, to enable long distance, over the horizon communications.
Other telecommunications schemes are in use or have been suggested that use satellite communications to provide a direct connection between the end user and the system. Examples of such telecommunications architectures include the IRIDIUM communications system, the Globalstar communications system, and the Teledesic communications system. Each of these systems utilizes a constellation of satellites to route messages from the end user or source subnetwork to the destination. The IRIDIUM communication system, for example, consists of a constellation of low earth orbit satellites for providing voice message and telephone service anywhere on the globe. The Teledesic communications system is a proposed system that envisions a constellation of satellites to provide direct access to internet broadband services via an antenna located at the end user facility.
Each of the satellite based communications networks incur the enormous cost of launching, maintaining and managing a constellation of satellites. This fact makes use of such satellite based systems costly when compared to alternative land based systems. In particular, each of the IRIDIUM, Globalstar, as well as the proposed Teledesic systems have encountered financial difficulties do to the high cost of providing such services and the reluctance of customers to pay the associated fees.
Although rare, satellite systems are also subject to interference and disturbance due to solar activity. One such event known as a geosynchronous magnetopause crossing actually places the satellite outside the protection of the Earth's Van Allen belt. The Van Allen belt is a magnetic field that serves to deflect harmful charged particles emanating from the sun away from the Earth. When conditions temporarily place the satellite outside this belt, the satellite is subject to harm from these particles disrupting communications and potentially damaging the satellite.
Other than line of sight microwave repeaters and satellite networks, the only other viable solution for over the horizon communications when landline connections are not available is high frequency (HF) radio. Due to the physics of HF radio wave propagation, HF radio can be used to transmit messages over the horizon for great distances. Use of HF radio, however, has some serious limitations. First, the HF signal is extremely subject to atmospheric interference which makes the signal “noisy” and of relatively low bandwidth. Second, the exact propagation of the HF transmission is highly dependent on the current atmospheric conditions. An HF frequency that works on one day to transmit between point A to point B may not work at all on another day. During periods of extreme solar events, HF communications may not be available at high latitudes at all.
Aircraft are ideal examples of a communications user that relies on each of the telecommunications networks described above to effect communications. Aircraft often utilize VHF or UHF communications for line of sight voice communications with ground support facilities such as air traffic control and flight services. Aircraft also use VHF communications for voice and data messaging such as through the aircraft addressing and reporting system (ACARS), for intra-airline communications, datalink services and weather information uplinks.
VHF communications, however, are limited to line of sight. In the event line of sight communications cannot be established, the intended VHF communication cannot be completed. For this reason, many aircraft are equipped with additional communications systems such as HF and satellite communications systems (SATCOM). The inclusion of these additional systems to facilitate airborne communications increases aircraft operating costs since additional equipment must be purchased and maintained. Furthermore, such systems are frequently operated by proprietary vendors, such as ARINC, that charge access and user fees to handle communications traffic using such systems.
Various proposals have also existed for use of the cellular phone network aboard aircraft. Use of cellular phones is currently prohibited aboard aircraft because a large number of cells are visible to the airborne transmitter. A transmitted message from the airborne unit thus overloads the cellular network and disrupts cell phone operations for all users. One company, Aircell, has proposed a system whereby the transmitting power of the unit is modified by connecting the unit to an airborne interface. The interface moderates the output power of the cellular communications. However, use of this system additionally requires use of specialized additional equipment and associated cellular phone access fees. This system will also not work if the individual aircraft is not within line of sight of a cell.